Passive Inter-Modulation (PIM) refers to a series of unwanted (but related) frequencies called harmonics created when two or more frequencies pass through a nonlinear device or junction. PIM can occur in components of wireless communication systems normally thought of as linear, such as cables, connectors, and antennas, but which are installed improperly or degraded in quality. Such nonlinear components can generate spurious signals when subject to the high radio frequency (RF) powers found in cellular networks. When two or more transmit frequencies, for example transmitted by a base transceiver station (BTS) of a cellular network, pass through the nonlinear component, the transmit frequencies are mixed and create PIM.
For carriers and the BTS, PIM represents noise or interference. The noise level (or noise floor) largely determines BTS receiver performance. Lower PIM levels result in better, more efficient BTS operation. For cellular network users, PIM can result in a rise in audible noise, dropped calls, reduced signal area, early handoff, and other problems. When a problem is detected at a BTS, a tower crew is dispatched to the site to measure for PIM. Standard PIM measurements only provide PIM magnitude. Typically a PIM measurement is provided to the tower crew as a pass/fail result where a fail result indicates a source of unacceptably large PIM. Upon obtaining a fail result, and relying only on the pass/fail result, the tower crew often has to dismantle an installation to identify the PIM source. If the PIM source is located beyond the antenna, for example where an unrelated antenna re-radiates signals received from and transmitted by the antenna of the BTS, the PIM source may not be resolved even by replacing all of the components of an installation.
Distance-to-fault techniques, which are described for example in U.S. Pat. No. 8,058,880 to Bradley et al., titled “CALIBRATED TWO PORT PASSIVE INTERMODULATION (PIM) DISTANCE TO FAULT ANALYZER,” incorporated herein by reference, have recently been developed to quickly identify the location of a PIM source, reducing the cost of repair by reducing the number of components that must be inspected and/or dismantled to eliminate the PIM. Distance-to-fault techniques have also made PIM monitoring practical, allowing network operators to monitor PIM over time and address creeping problems before catastrophic failure occurs.
Intermodulation products from continuous wave (CW) signals, such as might be created by a PIM tester, appear as single frequency CW products. However, PIM created from modulated carriers requires more bandwidth than the fundamental signals themselves. As a result, PIM products can be very wide-band, covering wide swaths of frequencies. Problematically, base station receive bands can be very constrained in frequency range, limiting the distance resolution of the distance-to-fault technique. What are needed are systems and methods for improving distance resolution within constrained receive bands.